1. Field of the Invention
The invention relates to a laser-sintering powder containing a polyamide such as nylon-12 and titanium dioxide particles, a process for preparing the powder, and moldings produced by selective laser sintering of the powder.
2. Discussion of the Background
Very recently, a requirement has arisen for the rapid production of prototypes. Selective laser sintering is a process particularly well suited to rapid prototyping. In this process, polymer powders are selectively irradiated briefly in a chamber with a laser beam, resulting in melting of the particles of powder exposed to the laser beam. The molten particles fuse and solidify to give a solid mass. Three-dimensional bodies can be produced simply and rapidly by repeatedly applying and irradiating additional layers of the polymer powder.
The process of laser sintering (rapid prototyping) to produce moldings made from pulverulent polymers is described in detail in U.S. Pat. No. 6,136,948 and WO 96/06881. A wide variety of polymers and copolymers are disclosed for this application, e.g. polyacetate, polypropylene, polyethylene, ionomers, and polyamide polymers.
Nylon-12 powder (PA 12) has proven particularly successful for producing moldings by laser sintering, in particular for producing engineering components. Parts manufactured from PA 12 powder meet the high requirements demanded with regard to mechanical loading, and have properties close to the properties exhibited by parts produced by mass-production techniques such as extrusion or injection molding.
The PA 12 powders used in these processes have a median particle size (d50) of from 50 to 150 μm, and may be obtained as described in DE 197 08 946 or DE 44 21 454, for example. In EP 0 911 142 it is disclosed that it is preferable to use a nylon-12 powder with a melting point of from 185 to 189° C., an enthalpy of fusion of 112 kJ/mol, and a freezing point of from 138 to 143° C.
Yellowing of moldings is a disadvantage with the polyamide powders currently used. This yellowing can appear on the finished component if the components are exposed to high levels of irradiation with UV-containing light, or to a long period of direct insolation.
However, a yellow tinge can appear during the actual laser sintering, resulting from the long period of severe thermal stress during this process. This effect is observed in particular when large amounts of returned powder are used, i.e. laser-sintering powder which has been previously applied to the molding but was not melted. Yellowing is often associated with impairment of the mechanical properties since aging of the material occurs. Examples of the noticeable effects of such aging include embrittlement, impaired tensile strain at break, and impaired notched impact performance.